PROJECT SUMMARY The plaques and tangles of Alzheimer?s disease (AD) were first described over a century ago; we now know for the first time that the proteins of these deposits are composed of A? and tau prions, respectively. Being able to measure these two prions using rapid human cell bioassays in conjunction with transgenic rodent models, we are now able to attack many areas of AD research that were previously inaccessible. We propose to expand our cohort of 114 brain samples from patients with AD and other neurodegenerative diseases to more than 1,000 specimens from brain banks worldwide, including equal numbers of aged-matched and gender-matched controls from cognitively normal subjects. We will survey the molecular profile of prion strains using a battery of cell lines and proteomics and relate these findings to detailed neuropathology and whole genome sequencing of every sample. Along these lines, we will also procure brain samples from upwards of 500 individuals with Down syndrome (DS) because these people all develop cognitive impairment and AD neuropathology at very young ages and have both A? and tau prions in their brains as we describe in this proposal. We will investigate the influence of the APOE genotype status on the levels of A? and tau prions in the brains of AD and DS patients, and perform mechanistic studies using human neural cells and humanized rodents. Importantly, we plan to expand our studies on the induction of tau prions by A? prions using novel human cell and rodent models on a background of different genetic risk factors. To identify the most relevant species of A? prions for these experiments, we will create a broad repertoire of synthetic A? prions composed of different isoforms informed by inherited AD mutations or genetic mosaicism as well as results from our proteomics data in human tissue. Since studies of neural exosomes collected from patients with AD or DS show increased levels of A? and tau proteins compared to controls, we plan to determine if these proteins are prions. If that proves to be the case, then it may be possible to develop a more biologically relevant blood test for AD, which would have enormous utility in the disease diagnosis and progression as well as the discovery of effective therapeutics. Our collaborations described in this P01 will allow us to rapidly define the structural events that underlie the molecular pathogenesis of AD as studied in this Project, which would seem critical to developing more effective AD therapeutics.